My mob grinder that I built last weekend! I mined out a small cave right next to my base and so I decided to make one.

entrance

inside

viewing area

spawning grounds

lava trap

This is where the water currents eventually takes the mobs to.

Any drops are carried here, outside.

The pressure plate activates the redstone torch, so I can just take a peak from my window to see if there's anything to be collected instead of walking up to it. I store all the drops in the chest on the left.

Redstone torch setup. All it is is a vertically-transmitted NOT gate.

And that's basically it. If you want to build one yourself, I can post more pictures of how I make the water currents, if you want.

Whoa. You must have some free time. That's some serious thought right there.

Not really. There's much bigger mob grinders that others have built that laugh at mine - some were done without cheating. Honest!

JakeAuditore:

Nice sig. XD

Post more pics! I'm totally copying this!

Okay, here you go!

The first thing you want to do is mine out an empty area that looks something like this:

Then place blocks in them at least 2 blocks high, so the mobs won't be able to jump out. Leave 2 block spaces for the waterways like I did here. You can leave more spaces in between the "spawning platforms" for more waterways.

And here is how you make the actual waterways. Just make sure you use either pressure plates or signs (not ladders!) in between each waterway so the water blocks don't merge with each other.

Arrange each waterway to lead to one place, the actual grinder. I'm sure you can figure that out on your own.

Here's how the grinder is built:

2 lava buckets on a 3x2 platform like so. Also, throw on 2 signs or ladders in this side of the platform like this so the lava doesn't fall below into the water.

The mobs eventually come in and burn to death, but their drops are
spawned at their feet, so their drops aren't burned in the process.
It's also important to have the low ceiling there so the mobs won't be able
to jump and burn their drops.

The last water way takes the drops to wherever you want them to. Block it off with a pressure plate like so. If you want a redstone torch to tell you if there's drops to be collected or not, here's the more simpler setup - redstone below pressure plate, redstone torch on the side connected to redstone, and redstone torch above that:

Good luck with the mob grinder!

edit: Here's the loot so far, but a bunch of it was from actually killing the mobs at night, hence the almost broken diamond sword.